Such cells are described for example in International Application No. WO 89/11165. (Hasvold 3). The invention relates in particular to a method for use with a seawater cell which is based on the reaction between oxygen, water and a metal anode, when it is submerged in seawater and connected to a load. The anode may be magnesium, aluminum or some other metal or metal alloy which is electro-negative with respect to the cathode.
The principal sources of deterioration for seawater batteries with long lifetimes are (a) anode depletion, (b) biofouling, and (c) development of a calcareous layer on the cathode. Anode depletion is obviously the factor which ideally should limit the life of the cell. Biofouling is dependent on season, water depth and the presence of sufficient nutrients in the seawater. In absence of light, and in only moderately polluted water, biofouling is not a problem over a time period of years.
The third source of deterioration (c) has, however, proved to cause the greatest problems. The object of this invention is therefore to reduce the rate of formation of a calcareous deposit on the cathode and substantially increase the lifetime of such seawater cells. To achieve this object we have found a method with involves measuring various cell parameters during cell operation and under varying load conditions according to certain criteria which will be clarified in the following. p Electrical circuits which supervise seawater batteries are known. In U.S. Pat. No. 3,470,032 there is shown a circuit where the detected voltage is used to control the water flow through the battery. In U.S. Pat. No. 3,012,087 and 3,542,598 there are suggested to use the detected battery voltage to vary the inlet of fresh seawater relatively to the recirculated water volume, whereby also the water temperature and the salt content are controlled. In the mentioned U.S. Pat. No. 3,012,087 there is also suggested that the battery voltage can be controlled by varying the water flow or by varying the water temperature by means of a heat exchanger.
In the mentioned publications, in particular the last mentioned, the load vary freely, while the voltage is maintained constant by varying different factors as e.g. the flow of water. With our seawater cells it is not practical to vary these parameters.
To use a control valve as mentioned in U.S. Pat. No. 3,470,032 to control in oxygen based seawater battery will be meaningless because too high water flow will not harm the battery, and no valve is better than an open valve. The same conclusion can be drawn with regard to U.S. Pat. No. 3,012,087. The circuit can of course activate a pump or propeller, but as previously mentioned the water requirement is so large that the natural convection caused by ocean currents is preferable, because a considerable part of the battery output would be used to pump water. Furthermore the reliability of such a battery will be reduced compared to a battery with no movable parts.
In U.S. Pat. No. 3,607,428 the water level within a battery compartment is varied by means of a valve in dependence of the battery voltage. The water flow requirements leads to the conclusion that the principles of U.S. Pat. No. 3,607,428 cannot be used for seawater batteries based on dissolved oxygen.
The control systems mentioned in the above patents are not considered suitable for eliminating or greatly reducing formation of calcareous deposits on the cathodes.
In DE OS 3 417 481 is described a battery connection where the load is disconnected when the battery voltage falls below a certain value. In UK Pat. No. 1 155 263 is described a battery connection for a truck where the battery can only be connected to a small load (driving) when the battery voltage falls below a certain value, while a larger load (lifting) is prevented.
Both these publications describe electrical circuits which reduces the load on accumulators, i e rechargeable (secondary) batteries, if the accumulator voltage becomes lower than a certain value. The reasoning for using such circuits in these two cases, is to secure the accumulators and contactors of the trucks, respectively, against overload and damage and to prevent fire. The damage referred to by overloading the accumulators, i e deep discharge, would be a reduction of the maximum number of obtainable cycles, i e the maximum number of charging and discharging cycles that an accumulator could normally take. It is well known that deep discharge reduces the lifetime of lead acid accumulators.
Primary batteries with which the present invention is concerned cannot be electrically recharged as the accumulators dealt with in the mentioned DE and UK patent specifications. In primary batteries there are other processes and parameters than cycling which in principle determine the life of the battery, namely the amount of active material (anode and cathode material) present in the battery and the obtainable rate of consumption of active material. It will normally be meaningless to use the mentioned circuit with primary batteries.
The present invention relates to seawater batteries which are primary batteries, the life of which in principle is determined by the amount of anode material available. The oxidant, (oxygen dissolved in seawater) flows freely through the battery and is reduced electrochemically at the battery cathode. The amount of oxidant available does not limit the lifetime directly.
In U.S. Pat. No. 3,959,023 there is described a power supply comprising a seawater battery of the magnesium silver chloride type. With such batteries there is formed a certain amount of sediments or slurry which after a certain time impairs the operation of the battery. The battery is connected to a light and/or a heavy load. A hydraulic pump is arranged in connection with the battery in order to pump the seawater electrolyte through the battery when it is heavily loaded.
A seawater cell based on oxygen dissolved in seawater requires a flow of seawater which is some 30,000 times larger than required for a silver chloride battery. This is the reason why seawater batteries based on naturally available dissolved oxygen is used for power sources where the power output is low and the discharging time is long, in order of months and years, while silver chloride batteries are used in systems (torpedoes, sonar buoys, emergency lights) requiring high power output for a short time. Due to the high rate of water exchange required, the batteries must have an open structure making series connection of cells in these batteries rather difficult, whereas series connections are common for silver chloride batteries.
Under normal seawater cell operation, i.e. when loading a cell, oxygen is reduced at the cathode according to the reaction: EQU O.sub.2 +2 H.sub.2 O+4e.sup.- =4 OH.sup.- ( 1)
This galvanic reaction consumes electrons, oxygen and water and produces hydroxyl ions which lead to an increase in the pH at the surface of the cathode: This alkalization of the seawater at the cathode surface can lead to a precipitation of a calcareous deposit of magnesium and calcium hydroxides and carbonates from calcium and magnesium salts dissolved in the seawater according to: EQU Mg.sup.++ +2 OH.sup.- =Mg(OH).sub.2 ( 2) EQU and EQU Ca.sup.++ +HCO.sub.3.sup.- +OH.sup.- =CaCO.sub.3 ( 3)
This formation of a calcareous layer is well known and considered beneficial in connection with cathodic protection of metal structures in seawater as it reduces the current necessary to protect the structure. Once formed, these slightly soluble salts do not easily redissolve. This effect is accordingly detrimental to seawater cell cathodes.
The rate of OH.sup.- production on the surface increases with the current density according to reaction (1). This leads to an increase in pH at the cathode surface. There will be a diffusion layer around the cathode and the increase in pH at the surface also increases with the thickness of the this layer. For a given cathode geometry, the thickness of the diffusion layer decreases with increasing seawater velocity. Thus either a high current density, resulting in a high rate of formation of hydroxyl ions or a low water velocity resulting in a low rate of transport of hydroxyl ions away from the cathode surface, leads to a pH increase at the cathode surface. This increase in pH is low and limited upward by the low concentrations of oxygen in the seawater. If however the cathode potential is so low that reduction of water to hydrogen: EQU 2 H.sub.2 O+2 e.sup.- =H.sub.2 +2 OH.sup.- ( 4)
takes place simultaneously, there is no such limitation on the upper pH, as the supply of water is nearly unlimited. In this case, the probability of formation of calcareous deposits on the cathode is significantly increased.